Charge amplifiers are required in particular in connection with piezoelectric sensors, since these output their measured values in the form of charges. Such sensors detect for example forces, pressures, accelerations, expansions, moments and related physical phenomena. Once such sensors are mounted on movable parts, for example on wheels of vehicles, the measurement signals are digitalised, in order to be able to be transmitted to a stator by means of contact-free transmission. For this purpose, the determined measurements are typically amplified in a charge amplifier and, by means of analogue-digital converters (A/D converters), converted until they are finally transmitted.
As a result of unavoidable interference currents at the amplifier input, which have the same effect as the currents originating from the changes in the charge Q, the amplifier output voltage moves from its original value; it drifts. In order to reduce this interference effect, a resistor is often connected in parallel with the charge amplifier, which limits the increase in the output voltage due to drift to an acceptable level. The resistor also acts on the measurement signal in a similar manner, however. The lower cut-off frequency of the charge amplifier therefore also often increases to values which can no longer be tolerated. If the measurement procedure only detects a brief single event, this interference effect can be counteracted by activating a reset switch shortly before the measurement procedure. In the case of longer measurement procedures, the choice of the value of resistor frequently leads to an unsatisfactory compromise between a resulting lower cut-off frequency and the residual drift of the charge amplifier. In addition, in the case of contact-free transmission of the measured value, for example with a moving measurement object, additional effort is necessary to activate the reset switch from of the fixed electronics.